The present invention relates to a method for packaging adhesives, and more particularly to a method for packaging hot melt adhesives in a pan and the resulting package formed thereby.
Hot melt adhesives are substantially solid at room temperature, but are applied in a molten or flowable state. Typically, hot melt adhesives are supplied in the form of solid blocks, pillows or pellets contained within a package that is meltable together with and blendable into the molten adhesive composition itself just prior to application. However, providing hot melt adhesives in these forms has unique problems, especially if the hot melt adhesive is pressure-sensitive. Since such substances are inherently sticky or soft at room temperature, there are problems associated with handling and packaging. Regardless of the form in which it is provided, a pressure sensitive adhesive not only sticks or adheres to hands, mechanical handling devices and to itself, but it also picks up dirt and other contaminates. In addition, adhesives with relatively low softening points will tend to flow or block together into a single solid mass rendering such adhesives difficult to be handled and/or packaged. Additionally, pressure sensitive formulations may deform or cold flow unless supported during shipment.
Many different approaches have been tried to package pressure sensitive hot melt adhesives. For example, U.S. Pat. No. 5,806,285 to Rizzieri teaches a method wherein adhesive is cast in a mold to form blocks. The mold has a plurality of holes formed therein and is lined with a thin film of plastic material which is vacuum thermoformed onto the inner surface of the mold. After filling the mold with adhesive, the free top surface is covered with a thin film of plastic material which is heat sealed to the film lining the interior of the mold. The mold containing the adhesive which is now enveloped by the film is then air cooled prior to removing the packaged adhesive from the mold. The major disadvantage of this process is that it cannot be water cooled due to the openings in the mold. The openings in the mold are necessary for the vacuum forming operation, and any attempt to water cool the mold would result in the adhesive floating out of the mold since hot melt adhesives are generally less dense than water. Due to the necessity of air cooling, the Rizzieri method is extremely slow in commercial production and requires a tremendous amount of time and space. In addition, since air is a relatively poor heat sink, this limits the temperature at which the hot melt adhesive can be dispensed into the mold. If the adhesive is dispensed into the mold at too high of temperature, it will melt the film. Thus, the Rizzieri technique is relatively slow and as such has limited applications.
Another process using molds is taught in U.S. Pat. No. 5,401,455 to Hatfield et al. The Hatfield et al patent teaches a method for packaging hot melt adhesive compositions using a solid mold in the form of a pan which has its outer surface in contact with a refrigerant gas or liquid heat sink. Hatfield et al teaches that when molten hot melt adhesive is poured into a cavity of the mold lined with film, the adhesive is fused to some degree with the film. According to Hatfield et al, this in turn improves later mixing of the film with the adhesive. However, a major disadvantage of Hatfield et al is that it is extremely difficult to consistently line the inner surface of a solid pan-like mold with a film so that the film does not wrinkle, crease or create voids between the film and the inner surface of the mold. If a continuous roll of film is used, the slightest movement of the film would cause the film to wrinkle resulting in voids or gaps between the film and the inner surface of the mold. It is desirable to avoid such gaps as they can cause burnthrough of the film. Thus, once again, the Hatfield et al method is extremely slow in commercial production, and has numerous technical problems that are difficult to overcome.
Yet another process utilizing a mold is disclosed in U.S. Pat. No. 5,715,654 to Taylor et al. In this process, Taylor et al teaches lining a rigid mold with a thermoplastic film which can be vacuum formed into the mold. However, if it is vacuum formed, the same cooling issues exist as in the Rizzieri method discussed above. In an attempt to speed up cooling, Taylor et al teaches that the center of the adhesive mass in the mold should be less than 1 inch from the nearest surface of the mold. The major disadvantage of such a mold is that it would produce a very small unit of adhesive. It would be preferable to have a method which would produce larger units such as blocks of adhesive. In addition, since there is no water cooling, the adhesive in Taylor et al would have to be dispensed into the mold at a relatively low temperature to prevent the film from melting. Again, since Taylor et al does not use water as a cooling medium, Taylor et al's process would be a very slow method, and thus would have limited commercial value.